The present invention relates to a torque distribution control system for a four-wheel drive motor vehicle having a central differential in a form of a complex planetary gear device.
In the four-wheel drive motor vehicle, a torque distribution ratio of front and rear wheels is determined in accordance with a dynamic load ratio of the front and rear wheels and with movement of the center of gravity of the vehicle at acceleration so that the power of the an engine may be most effectively used.
In the four-wheel drive vehicle based on front-drive with a front-mounted engine, the ratio of the front torque T.sub.F to the front wheels and the rear torque T.sub.R to the rear wheels is approximately 50:50 (TF:TR.apprxeq.50:50). In the four-wheel drive vehicle based on rear-drive with a front-mounted engine, the ratio of the front torque T.sub.F to the rear torque T.sub.R is approximately 40:60 (TF:TR.apprxeq.40:60). A central differential comprising bevel gears is provided in the former system in which the output torque of the engine is equally distributed, and a central differential having a simple planetary gear device is provided in the latter system.
The former system ensures safe driving on a slippery road. If a differential lock device is provided for locking the central differential, the driveability of the vehicle is improved. However, when the vehicle makes a turn at high speed under the differential lock condition, all of four wheels may slip (i.e. the vehicle spins) at the same time, causing difficult driving.
In order to ensure driving stability of the vehicle, the torque to the rear wheels is set to a value larger than that to the front wheels by arranging the central differential comprising the simple planetary gear device, so that first the rear wheels may slip. Thus, the vehicle can be safely driven by the front wheels at a small torque while the rear wheels do not drive.
Japanese Patent Application Laid-Open 63-176728 discloses a four-wheel drive motor vehicle in which a central differential comprising a simple planetary gear device is provided. The output of the transmission is transmitted to a carrier of the planetary gear device. The torque is distributed to the front wheels through either a sun gear or a ring gear and to the rear wheels through the other gear. The torque to the front and rear wheels is unequally distributed at a ratio determined by the difference between the pitch circles of the sun gear and the ring gear. A fluid operated multiple-disk friction clutch as a lock device is provided for controlling the differential operation. The clutch comprises a drive drum and a driven drum each having multiple-disks, and a piston oil chamber formed between the drive drum and a piston. An electronic control system and a hydraulic control system are provided for controlling the pressure of oil. When oil is supplied to the piston chamber, the piston is urged by the pressure of the oil to push the disks of the drive drum. Thus, the opposite disks are engaged to produce a torque for restricting the differential operation.
Japanese Patent Application Laid-Open 60-159444 discloses a system for controlling a differential without electronic and hydraulic control systems. The differential comprises a worm gear and a worm wheel provided between right and left wheels or between the front and rear wheels for permitting the differential operation when a motor vehicle makes a turn. When the vehicle travels on a straight road, the differential is locked because of the irreversible function of the worm gear.
In the former system, the system is complicated in construction and becomes large in size because of the hydraulic control system and the electronic control system including a solenoid operated control valve for operating the clutch. Since the pressurized oil is supplied to the clutch for actuating the clutch to restrict the differential operation, a time lag inevitably occurs in the clutch operation. If the length of a hydraulic circuit in the hydraulic system is long, the response in operation is deteriorated, which decreases driveability of the vehicle. Furthermore, the differential operation is largely changed in accordance with fluctuation of the friction between the drive disks and driven disks. Therefore, it is desired to accurately control the pressure of the oil in accordance with driving conditions of the vehicle.
In the latter system, since the worm gear is used, the structure of the differential is complicated so that manufacturing cost increases. Further, since the worm gear device has a large sliding resistance, transmission efficiency reduces and wear of the gears increases. The system can not use the same oil of the transmission system the actuating oil so that the system is not provided in the transmission system.